3.1. General characteristics of the technologies and where they are applied

Of the terrace technologies described in WOCAT, 50% originate from Asia, 33% from Africa, 11% from Europe, and 6% from South America, with a notable presence of countries such as Ethiopia or China. The technologies are generally applied in small areas: 30% have an area between 0.1 and 1 km², and 70% occupy less than 100 km² (only 27% have an area > 1,000 km²).

Soil degradation in the areas where the terraces are applied usually occurs due to a combination of different causes or drivers: (i) indirect (land tenure, population pressure, education, inputs and infrastructure, poverty); (ii) human induced (deforestation, soil management, disturbance of the water cycle, gradual abandonment of mountain agriculture); (iii) indirect and natural (extreme rainfall, steep mountain terrain); and (iv) indirect and naturally induced. In relation to the terraces, the main type of degradation reported in WOCAT is degradation due to water erosion (75%), followed by the combination of water erosion and chemical soil deterioration (21%) and the combination of water and wind erosion, representing only 4% of the cases. Hence, the main technical functions of terraces are the control of runoff, the reduction of the slope, the increase in water infiltration and soil cover, the incorporation of organic matter into the soil, and the increase in soil fertility. The objective of the interventions is predominantly the mitigation of degradation processes, followed by prevention and rehabilitation. Farmers are aware of the need to implement measures to prevent soil degradation, so in 50% of cases the origin of the technologies is a local initiative, while the other 50% have an external origin.

In 64% of the cases reported, terraces are structural conservation measures, in 14% they are combined structural/vegetative measures, and in 8% a combination of agronomic/vegetative/structural measures. The typology of the terraces is diverse: terraces made with stones predominate, representing 36%; followed by those built with earth and vegetation (25%) and earth alone (22%). The remaining 6% correspond to terraces built with diverse materials such as tires or cement bags filled with earth. The choice of the type of terrace depends on several factors, such as the type of soil, slope gradient, availability of stones, availability of labor, or cost. The dimensions of the terraces vary depending on the technique used and the type of slope. On steep slopes, high terraces predominate (Ethiopia, Peru, Syria). The technique used is usually related to the purpose or to the type of crop to be planted.

From an environmental point of view, 64% of the technologies reported have been developed in semi-arid climates, 19% in sub-humid, and 17% in humid. The predominance of terraces in semi-arid regions is due to the irregularity of rainfall and the serious problems of soil degradation in these environments. Regarding the pre-existing morphology, hillslopes account for 64% of cases. The predominant slope is hilly (16-30%), followed by steep (30-60%) and rolling (8-16%).

The soils from semi-arid regions where the terraces are implemented are mostly shallow (< 50 cm), while those from more humid regions can exceed 120 cm in depth. Regarding the texture, most (72%) have a medium texture (loam), 19% have a coarse (sandy) texture, and the rest have a fine texture (clay). Soil fertility is low or very low in 60% of cases and average in 37%, while only 3% of the soils have high fertility. The superficial soil organic matter content is consistent with the soil fertility, being low (< 1%) in 60% of the cases analyzed and moderate (1-3%) in 40% of them. Regarding the hydric properties of the soils, the drainage capacity is moderate-high in most cases and the soil water storage capacity is moderate-high in 50% of them.

3.2. Cost-benefit ratio of implementation and maintenance

The cost-benefit ratio of terrace implementation and maintenance is an important aspect for farmers. Analysis of the WOCAT database shows that if the implementation costs of terraces are low, their maintenance costs are usually high and vice versa. This is because the terraces built with earth are cheap to build, but have to be repaired continuously, while the cost of building terraces with stonewalls, that require specialized personnel, is much higher but they last longer and have very limited maintenance costs. There are records of centuries-old terraces built with stone from Peru, Syria, and Israel (Ore & Bruins, 2012).

Farmers consider the establishment costs of terraces to be negative in the short term (1 year), but positive and very positive in the long term (10 years). Regarding maintenance, the costs are perceived as positive and slightly positive in the short term, and positive and very positive in the long term (Figure 1).

Figure 1: Cost-benefit ratios for the establishment and maintenance of terraces in the short term and long term. Source: own elaboration from WOCAT database. Figura 1: Relación coste-beneficio para la implantación y mantenimiento de terrazas a corto y largo plazo. Fuente: elaboración propia a partir de la base de datos WOCAT.

3.3. Impacts of terrace construction

Socioeconomic and production impacts. The most important positive impacts derived from the construction of terraces are increases in crop yield, fodder production, and farm income. Other benefits mentioned, although less frequently, are the simplification of farm operations, increased animal and wood production, reduced demand for irrigation water, more efficient irrigation and fertilizer use, and a lower risk of crop failure. In a few cases negative effects are reported - such as increased labor constraints, the need for greater agricultural inputs, loss of land, access constraints, and reduced crop production, since the terrace occupies part of the previously productive land. The changes with lower impact are the increased supply of irrigation water, breakage of the structure of the topsoil, hindering of farm operations, increased economic inequality, and, in some cases, reduced animal output.

Sociocultural impacts. The most notable benefits are the improved knowledge of soil conservation and erosion, strengthening of community and national institutions, and improved food security. In three cases the improvement of cultural opportunities and of the situation of certain disadvantaged groups is mentioned. Regarding negative impacts, sociocultural conflicts are mentioned in four cases and the loss of recreational opportunities in one.

Ecological impacts. A high positive impact is associated with the reduced soil loss, increased soil moisture, improved soil cover, and reduced surface runoff, together with increases in the soil fertility and biodiversity. To a lesser degree, the collection of surface runoff improves, the loss of excess water by drainage is diminished, the soil organic matter increases, and the risk of adverse events is reduced. A small number of cases report reductions in the evaporation, soil crusting/sealing, and wind velocity, and increases in the aboveground biomass, plant diversity, quantity of water available, carbon sequestration, and groundwater recharge. The main ecological disadvantages, mentioned in three cases, are related to reduced river flow and an increased number of niches for pests. In two instances, increased fire risk, increased risk of waterlogging, and reduced biodiversity are cited.

Off-site impacts. The positive effects, with high or very high impacts, are related to reduced flooding and siltation downstream. In second place is increased streamflow in the dry season, followed by reduced surface water pollution, reduced damage to infrastructure in the fields of neighbors, and a decline in wind transported sediments. To a lesser extent, there is increased groundwater recharge, the sediment yields in the valley bottom decrease, and the water buffering/filtering capacity improves. By contrast, reduced sediment yields are considered a disadvantage in six cases, in four of which reduced river flows are also evaluated negatively.

3.4. Ecosystem services provided by terrace construction

Based on the impacts that derive from terracing, we identified a series of provisioning, supporting, regulating, and cultural ES provided by terraces.

Provisioning Ecosystem Services. Often, the term provisioning ES refers mainly to the supply of food, fiber, fuel, and water, but we also include the supply of land available for production, since this was mentioned in two cases. Increases in the provision of food and, as a consequence, improvements in food security stand out, as do, to a lesser degree, the increased animal output, reduced risk of crop failure, and increased fodder production. In some cases, terraces have improved the quality and quantity of the available water. The water quality rises due to improved buffering and filtering, thereby reducing groundwater and river pollution. The quantity of water available increases due to a reduced demand for irrigation water, thus enhancing the harvesting of water and its availability. Wood production constitutes a provisioning ecosystem service in six of the technologies present in the WOCAT database and land supply does so in two (Figure 2).

Figure 2: Provisioning ecosystem services provided by terraces. Impact: H = high, M = medium, and L = low. Source: own elaboration from WOCAT database. Figura 2: Servicios ecosistémicos de aprovisionamiento proporcionados por las terrazas. Impacto H = alto, M = medio y L = bajo. Fuente: elaboración propia a partir de la base de datos WOCAT.

Regulating Ecosystem Services. In order of importance, these are: (i) reduction of natural hazards, (ii) reduction of erosion, (iii) water regulation, (iv) climate regulation, (v) biological regulation, and (vi) health (Figure 3). By reducing runoff and erosion, the floods, sedimentation, and damage to infrastructure downstream and in neighboring fields are reduced. Water regulation involves increased soil moisture, decreased runoff, reduced evaporation, water retention, and recharge of aquifers. Climate regulation is achieved by the sequestration and stabilization of carbon in soils. Regarding biological regulation and health, terraces are less important, although in some cases increases in the diversity of plants and animals and in the abundance of beneficial species, and an improvement in the functioning of the ecosystem, are mentioned.

Figure 3: Regulating ecosystem services provided by terraces. Impact: H = high, M = medium, and L = low. Source: own elaboration from WOCAT database. Figura 3: Servicios ecosistémicos de regulación proporcionados por las terrazas. Impacto H = alto, M = medio y L = bajo. Fuente: elaboración propia a partir de la base de datos WOCAT.

Supporting Ecosystem Services. These ES, identified in fewer cases, refer to: (i) genepool protection, due to enhancement of (soil) biodiversity, and (ii) nutrient cycling, due to increased soil fertility, aboveground biomass, and nutrient cycling recharge (Figure 4).

Figure 4: Supporting ecosystem services provided by terraces. Impact: H = high, M = medium, and L = low. Source: own elaboration from WOCAT database. Figura 4: Servicios ecosistémicos de soporte proporcionados por las terrazas. Impacto H = alto, M = medio y L = bajo. Fuente: elaboración propia a partir de la base de datos WOCAT.

Cultural Ecosystem Services. Cultural ES include the potential for recreational activities, education, and strengthening of institutions. It is worth pointing out the improved knowledge of soil erosion and conservation (64% of cases) and the strengthening of community institutions (42% of cases) (Figure 5).

Figure 5: Cultural and amenity ecosystem services provided by terraces. Impact: H = high, M = medium, and L = low. Source: own elaboration from WOCAT database. Figura 5: Servicios ecosistémicos culturales y recreativos proporcionados por las terrazas. Impacto H = alto, M = medio y L = bajo. Fuente: elaboración propia a partir de la base de datos WOCAT.

4.1. Provisioning Ecosystem Services

Our analysis demonstrates that many terraces are designed to provide a larger area for cultivation on slopes, spaces that would otherwise be very difficult to cultivate, while guaranteeing long-term agricultural cultivation (Figure 2). Thus, terraces contribute to greater agricultural production potential. Haas et at. (1966) highlighted their contribution to the increase in the provision of food and water, indicating that there is greater soil moisture in the terraces, contributing to higher crop yields. Hammad et al. (2004) reported that terraces contributed to increased wheat production, while Hammad & Børresen (2006) found a 2.5-times higheryield with terraces. In addition, several studies indicated how terraces, especially those made with stones, favor drainage, water conservation (Chow et al., 1999), and groundwater recharge (Liu et al., 2004). Schiettecatte et al. (2005) and Rashid et al. (2016) considered terraces very effective water harvesting techniques.

4.2. Regulating Ecosystem Services

Terraces reduce the slope gradient and surface runoff, favoring the infiltration of water so that soil erosion processes decreases. For this reason, terraces deliver numerous regulating ES to the owners of the land and to society (Figure 3). As erosion decreases and water retention increases, the production of food, fiber, and wood together with the quantity and quality of the water available improve and income increases. This retention of soil and water minimizes natural risks (e.g. floods), reducing construction and maintenance costs for infrastructure like irrigation channels or reservoirs, especially in regions affected by irregular or intense seasonal rainfall (Sandor & Eash, 1995). This function of water retention in soils and reservoirs will be increasingly important if the climate change projections characterized by a decrease in precipitation and increased droughts and torrential rains materialize (Eekhout et al., 2018). Regarding fire risks, the agricultural use of the terraces configures areas with scarce vegetation and a marked horizontal discontinuity of fuel, thus reducing the risk of fire and its propagation (Reynes Trias, 2006). However, García-Ruiz et al. (2013) commented that abandoned terraces in which plant colonization has been rapid can be more prone to fires.

The importance of the regulating ES is reflected in the large volume of research highlighting the effectiveness of terraces regarding erosion and flood reduction (e.g. Beach et al., 2002; Gebremichael et al., 2005; Zhang & Li, 2014; Chen et al., 2017). Mekonnen et al. (2015) identified terraces as sediment retention traps and Wheaton & Monke (1981) considered them the best measure for erosion control. In a sub-humid climate in India, Sharda et at. (2002) verified experimentally how terraces are very effective in reducing runoff and soil loss: by more than 80% and 90%, respectively, compared to the system without terraces. In addition, Zhang & Li (2014) noted how terraces in northeastern China reduced the rate of soil erosion by 16% for the entire slope and recommended that traditional farming practices be changed to terraced cultivation. Terraces can reduce water erosion significantly, provided they have been properly planned, constructed, and maintained; otherwise, they can cause land degradation and increase erosion (Dorren & Rey, 2004; Romero Díaz et al., 2007). The important role of terraces in water regulation has been shown on numerous occasions, from the ancient Mayan terraces (Beach et al., 2002) to those of Pakistan (Rashid et al., 2016), China (Liu et al., 2004), and Spain (Reynes Trias, 2006).

One consequence of the regulation of runoff, increased infiltration, and enhanced recharge of aquifers (Liu et al., 2004) is the reduction of floods (Wei et al., 2016). In fields of Bohemia, Kovár et al. (2016) simulated the influence of terraces on runoff using the KINFIL model and verified the effectiveness of terraced systems in flood mitigation and the resulting soil erosion. Dumbrovsky et al. (2014), in the Czech Republic, modeled the impacts of different terraces and performed a cost-benefit evaluation of the effect of terrace design on the sedimentation of water courses and potential damage in urban areas as a consequence of floods. They concluded that the average potential damage exceeded € 2.4/m². Thomas et al. (1980) highlighted the regulation of water by terraces. The higher carbon sequestration in terraced soils and its relevance to climate mitigation was highlighted by Antle et al. (2007). Bocco & Napoletano (2017) stressed the important role of terraced agriculture in climate change adaptation in Latin America.

4.3. Supporting Ecosystem Services

Supporting ES mainly concern the improvement of the nutrient cycle and biodiversity (Figure 4). The formation of terraces by filling and plowing also helps to form zones with a high content of nutrients (Stanchi et al., 2012). Liu et al. (2011) demonstrated how terracing in the Loess plateau in China stores and retains large volumes of water, promoting more favorable interactions between water and fertilizers. Damene et al. (2012) concluded that terracing in Ethiopia reduced the loss of nutrients by erosion, but indicated that it is unlikely that terracing alone will improve soil fertility. This requires terracing combined with integrated nutrient management. In an extensive review, Stanchi et al. (2012) concluded that soil fertility often declines following the abandonment of terraces, if erosion processes intensify. Kagabo et al. (2013) analyzed the fertility of soils in terraced and non-terraced areas in Rwanda, and verified that soil fertility was higher in the lower parts of the terraces, which had a higher organic carbon content (57%) and higher phosphorus availability (31%) than the higher parts of the terraces. Arevalo et al. (2017) showed that the soil quality in terraced fields on the island of Lanzarote (Canary Islands) was better than in non-terraced fields. Soil conditions are often improved in consolidated terraced terrain, in comparison with recently terraced landscapes, as found in vineyards in the Northeast of Spain (Ramos et al., 2006).

Regarding biodiversity, terraced cultivated landscapes often allow the conservation of varied plant and animal communities. According to Bragg & Stephens (1979) and Wei et al. (2016), terraces can benefit the restoration of vegetation and improve the biodiversity due to the higher soil moisture and nutrient content. Egea Fernádez & Egea Sánchez (2010) mentioned how some birds build their nests on the terraces of rainfed arboreal crops, and Kosulic et al. (2014) found that vineyard terraces can serve as a refuge for endangered spiders. Following abandonment, on occasions, the diversity of plants is greater and even the stone-walls themselves can house a large number of plant and animal species (Tokuoka & Hashigoe, 2015).

On the other hand, terraces can also have negative impacts, especially if poorly planned, constructed, or maintained. For example, through severe land leveling, subsoiling, and the use of heavy machinery, terracing can reduce the content of organic matter, the water retention capacity, and the stability of the aggregates. In this respect, Bazzoffi et al. (2006) showed that soil losses due to landslides were greater in Italian terraced vineyards, and Romero Díaz & Belmonte Serrato (2008) found that the terraces built for afforestation destroyed the original soil, leaving poor soils that lacked structure and nutrients.

4.4. Cultural Ecosystem Services

Terraces have an important cultural and ethnological value throughout the world. The transformation of the environment by the application of traditional techniques has given rise to a set of knowledge, practices, and techniques of great value. These cultural landscapes form part of the identity and cultural diversity of many regions and are worth conserving and protecting. In addition, in many cases, they can become a tourism resource, taking into account their landscape, cultural, and historical values. Among the educational and scientific ES of the terrace technologies analyzed, the better knowledge of the erosion and conservation of soils acquired by farmers stands out, as does the strengthening of the community institutions (Figure 5).

The uniqueness of terraced landscapes is recognized internationally, as reflected in some cases by their status as UNESCO World Heritage Sites. This is the case, for example, of the terraces of Machu Picchu in Peru (since 1983), the rice fields of Ifugao in the Philippines (1995), Cinque Terre in Italy (1997), the vineyards of the Douro in Portugal (2001), and the Sierra Tramuntana on the Spanish island of Mallorca (2011), among others. These areas also stand out from the landscape point of view (Paoletti, 1999), which is why they constitute a tourism resource (Qiu et al., 2014). It is also worth mentioning that in November 2018 “The art of dry stone construction: knowledge and techniques”, with contributions from countries such as Croatia, Cyprus, France, Greece, Italy, Slovenia, Spain, and Switzerland, was declared part of the Intangible Cultural Heritage of Humanity by UNESCO.